1. Field of the Invention
This invention relates to tripeptides which possess pituitary growth hormone releasing activity.
2. Description of the Prior Art
Growth hormone, which is secreted from the pituitary, causes growth of all tissues of the body that are capable of growing. In addition, growth hormone is known to have the following basic effects on the metabolic process of the body:
1. Increased rate of protein synthesis in all cells of the body;
2. Decreased rate of carbohydrate utilization in cells of the body;
3. Increased mobilization of free fatty acids and use of fatty acids for energy.
A deficiency in growth hormone secretion can result in various medical disorders, such as some instances of dwarfism.
Various ways are known to release growth hormone. For example, chemicals such as argine, L-dihydroxyphenylamine (L-DOPA), glucagon, vasopressin, and insulin induced hypoglycemia, as well as activities such as sleep and exercise, indirectly cause growth hormone to be released from the pituitary by acting in some fashion on the hypothalmus perhaps either to decrease somatostatin secretion or to increase an unknown endogenous growth hormone releasing hormone or both.
Compounds which directly act on the pituitary to release growth hormone include prostaglandin E.sub.1 and E.sub.2, theophylline, and cyclic nucleotides. However, these compounds neither specifically release growth hormone nor are they believed to act at the putative growth hormone-releasing hormone receptors in the peripheral membrane of the pituitary cell to initiate growth hormone release.
In addition, under special conditions certain chemically defined peptides, e.g., vasopressin, thyroid-releasing hormone (TRH), luteinizing hormone-releasing hormone (LH-RH), .alpha.-melanocyte-stimulating hormone (.alpha.-MSH), glucagon, substance P, neurotensin; Met-enkephelin, .beta.-endorphin, cholera-enderotoxin, and basic myelin protein, act to release growth hormone from the pituitary. However, only TRH acts directly on the pituitary to elicit this response. Furthermore, the above listed peptides release other pituitary hormones and under most experimental conditions do not release growth hormone. For example, TRH does not release growth hormone in normal rats or in normal humans or from pituitaries of normal rats or monkeys. In vitro, TRH releases growth hormone, prolactin, and thyroid stimulating hormone (TSH) and in vivo TRH releases these hormones from bovine pituitary.
Vasopressin's induced release of growth hormone is considered to be due to a non-specific response to stress caused by administration of high dosages of vasopressin.
Accordingly it would be highly desirable to have a compound which directly acts on the pituitary under normal experimental conditions to effect the release of growth hormone therefrom. Such peptides would be useful in vivo as unique research tools for understanding how growth hormone secretion is regulated at the pituitary level and would also be useful in vivo to treat symptoms related to growth hormone deficiencies.